The field of the invention is flow meters for measuring the volumetric flow rate and the energy flow rate of gases in a pipeline.
The measurement of volumetric flow rate in gas pipelines has been the subject of research and development for many years.
In gaseous flows, the phenomenon of compression exists and has a large effect. It allows the number of molecules for a given volume to change with pressure and temperature as well as with composition. Therefore, it is desirable to make natural gas sales transactions either by mass, energy, or at standard pressure and temperature conditions. In the U.S., for example, the standard pressure and temperature of gas is stated as 14.73 psia and 60.degree. F. for many transactions. Delivery calculations state the flow is adjusted to correspond to these base conditions even though the actual gas in the transaction is probably at a different pressure or temperature. A piece of equipment designed to accomplish the task of converting a measured volumetric flow rate to a base volumetric flow rate at a defined pressure and temperature is referred to as a "volume corrector".
In the traditional method of gas measurement, a volume correction ratio Q.sub.b /Q.sub.f is determined from the pipeline flow conditions using the following relation: ##EQU1## where Q.sub.f is the measured volumetric flow rate of the pipeline gas through the pipeline, T.sub.b and P.sub.b are the base condition temperature and pressure (e.g., 14.73 psia and 60.degree. F.), T.sub.f and P.sub.f are the measured flow temperature and pressure of the pipeline gas in the pipeline, Z.sub.b and Z.sub.f are the supercompressibility factors at the base condition and the flow condition, respectively, and Q.sub.b is the base condition volumetric flow rate. Such a calculation is typically carried out in a flow computer.
Using the relation in Eq. (1) to compute base condition volumetric flow rate Q.sub.b requires accuracy in the measurement of the flow temperature T.sub.f and pressure P.sub.f. This requires that pressure and temperature sensors for monitoring P.sub.f and T.sub.f be calibrated frequently.
The supercompressibility ratio Z.sub.b /Z.sub.f in Eq. (1) is difficult to measure. One known way to measure the composition of the gas uses gas chromatography. In this method, the supercompressibilities, Z.sub.b and Z.sub.f, are estimated from either the virial equations of state, or from precalculated correlations such as NX-19 or the more recent Gerg Equations. Alternatively, a meter that measures heating value, relative density, %CO.sub.2 and %N.sub.2 can be used to calculate the ratio Z.sub.b /Z.sub.f. This is because the Gerg Equations in their short form allow calculation of the ratio Z.sub.b /Z.sub.f from these parameters.
Knowledge of the values of the virial coefficients of particular gas compositions is quite limited so calculation of supercompressibility from the virial equation of state is not always possible. The Gerg Equations and NX-19 correlation are mathematical models obtained by mapping known and measured properties. The Gerg Equations, in particular, are very good over a wide range of compositions. Use of the Gerg Equations, however, requires either a chromatograph or a special meter to measure the properties needed to solve the short form Gerg Equations, and both of these techniques are considered expensive. It has, therefore, been difficult to obtain accurate measurement of the supercompressibility ratio Z.sub.b /Z.sub.f in a cost effective manner.
In Vander Heyden, U.S. Pat. Nos. 5,307,668, and 5,323,657, these problems were addressed by providing a sampling device for sampling the pipeline gas and relating the mass flow rate of the sample to the pipeline gas. The methods and apparatus disclosed there overcame the problem of using inferred values. However, the technique of measuring volumetric flow rate of the sample gas at base conditions utilized the measurement of energy flow rate and heating content of the sample gas at these conditions, which involved combusting a sample of the gas.
The present invention is a further improvement in measuring volumetric flow rate of gas in a pipeline, which is responsive to the composition and the density of the particular gas flowing in the pipeline. The invention solves the problem of measuring the volumetric flow rate at pipeline temperature and pressure and provides a method for relating the volumetric flow rate measurement to base temperature and pressure without combusting the gas.